This invention relates to a railway wheel tread-profile.
In a railway vehicle design two main factors have to be considered. The first is the dynamic stability of the vehicle at various speeds throughout its operating speed range. The various vehicle masses, such as body, bogies, and wheelsets each of which comprises a pair of wheels solidly mounted on an axle, experience oscillations known in the art as "hunting". The maximum speed of the vehicle or critical speed is determined by the onset of unstable, undesirable wheelset hunting. This hunting is caused by the combination and resonance of the hunting caused in the wheelsets by their resilient suspension to the other vehicle masses and by the creep forces generated by the conical wheel treads on the rails. The hunting caused by creep forces is speed dependent. Increasing the conicity of the wheel-treads increases the creep forces causing hunting and therefore lowers the critical speed of the vehicle. The second factor is the ability of the vehicle to negotiate track curves. This curving ability is determined primarily by the ability of the wheelsets to follow the track curves. Optimally the wheelsets should perform a purely rolling motion in track curves without any contact between the wheel flanges and the rails. This requires steering forces to be generated by the conical wheel tread independently of the wheel flange and a suspension permitting the wheelset to yaw, i.e. rotate about a vertical axis through its center of gravity, to approach a radial position with respect to the track curves. Increasing the wheel-tread conicity improves the steering ability of the wheelsets because of the increased steering forces thereby generated within the limited gauge clearances (distance between wheel flange and track) available. Therefore, with regard to the conicity of the wheel-treads, there is a conflict between the requirements for hunting stability and increased vehicle speed and for a good curving ability of the wheelsets. For this reason conventional railway vehicles use a wheel-tread with a straight taper of the order of 1 in 20 as a compromise solution. A taper or conicity of 1 in 40 has also been used on special track. A problem with this approach is, however, that in service the wheel-treads wear which changes the effective conicity of the wheel treads so that hunting stability cannot be maintained in service.
In order to overcome the abovementioned difficulties it has been proposed to make the wheelsets self-steering by providing them with a high effective conicity to generate steering forces and by suspending the wheelsets to the other vehicle masses in such a manner that the wheelsets can yaw to assume a radial position in track curves. The increased tendency to hunting caused by making the wheelsets self-steering is counteracted by coupling the wheelsets in opposite senses which, ultimately, generates hunting stabilizing creep forces in the wheel/rail contact areas. This approach is outlined in U.S. Pat. No. 4,067,261. Moreover with this approach profiled wheel-treads are used, i.e. they do not have a straight taper, so that the profile does not change significantly with wear and hunting stability is retained in service.
With a conical (straight taper) wheel-tread the conicity remains virtually constant with lateral deflection of the wheelset relative to the track. In other words the conicity remains the same irrespective of whether the wheel runs centrally on the track or is deflected closer to one rail. If profiled wheel-treads are used, i.e. the running surface is formed of arcs or parts of a circle, then the radius of these arcs must be greater than the radius of the rail head and also, as a consequence, the effective conicity of the running tread will be greater than the inclination of the rail to the vertical. The effective conicity is given as an approximation by the following relation for wheel treads, the running zone of which is formed of a single arc: ##EQU1## where:
.gamma. is effective conicity,
.delta..sub.o is angle the rail is inclined to the vertical
R.sub.w is radius of the wheel-tread, and
R.sub.r is the radius of the rail-head.
In the art the term "profiled" is used in relation to wheels having a curved running zone and distinguishes such wheels from conventional wheels having a linear tread. With linear treads there is a linear change in rolling diameter as the wheel moves laterally on the rail and the taper formed on the tread defines this change. Since the tread has the form of a truncated cone this taper is referred to as "conicity". With profiled wheels, however, the curvature or tread profile, the rail-head radius, and rail inclination result in non-linear diameter changes and non-linear lateral changes in wheel/rail contact points on the field side of one wheel and the gauge side of the other for unit linear lateral movements of the wheelset (e.g. see FIG. 7 of the drawings). Thus, the change in rolling diameter per unit lateral movement of the wheelset is no longer linear, but the conical effect remains. For this reason and because profiled wheels are not strictly speaking conical, the term "effective conicity" is used in relation to such profiled treads. The relation given above is an approximation given by Wickens in a paper entitled " The Dynamics of Railway Vehicles on Straight Track; Fundamental Considerations of Lateral Stability", Proc. Inst. of Eng. 1965, Vol. 180 on page 4 equation 1.5, for a special case of profiled tread as stated, and is not applicable or reliable where the profile is not a single arc. A general definition of conicity that applies to linear and non-linear tread forms is that it is one-half the sum of the changes in rolling radius of the two wheels of a wheelset divided by the lateral deflection of the wheelset on the rails. This definition may be used to give a clearer picture of what effect the shape or profile of the running zone--as seen in a typical drawing--has on conicity or effective conicity. With straight taper wheel treads, the conicity is constant. With prior art type treads formed of an arc of constant radius, e.g. West German Pat. No. 862,458 the rolling diameter increases with lateral deflection and in use on a rail with a radiussed head the rate of increase of rolling diameter is substantially constant. The effect is a substantially constant effective conicity and one which is obviously higher than a straight taper. With "worn wheel" type prior art treads, the arc radius of the tread profile curve decreases (i.e. the curvature increases) towards the wheel flange so that in use the rate of change of rolling diameter increases with increasing lateral displacement. This has the effect of increasing the effective conicity progressively towards the wheel flange. The shape of a very worn wheel, e.g. U.S. Pat. No. 1,783,706 Emerson et al, which has a "hollow" profile must also be considered. The main characteristic of this profile is that there is portion of the running zone in which the rolling diameter in fact decreases with lateral displacement. This results in a negative effective conicity in this zone, i.e. the rate of change of rolling diameter becomes negative or less than zero in this zone. This negative conicity exists provided that contact of the wheel with the rail is maintained in this zone; if this is not so then there is an undesirable cut-off of the conicity. These previously mentioned "rates of change" will be evident to the eye on inspection of a longitudinal section through a wheel tread, e.g. as shown in the attached drawings. It has been found that such a progressively increasing conicity lowers the hunting stability of the wheelset as the wheelset appears to respond to the highest conicity which prevails when the wheelset is furthest deflected.
A further factor which must be considered in designing a wheel-tread is the wearing characteristic resulting from the form of the tread itself without consideration of the materials used for the wheel and rail. Contrary to normal expectations that an increase in contact area would necessarily decrease tread wear, research has now shown that the rate of wear of the tread can, in fact, increase when the shape of the tread approaches the shape of the rail head. This is because the wheel-tread/rail-head contact area increases to such an extent that there is a disadvantageous increase of the creep forces or slip of the wheel on the rail which in turn increases the wear rate significantly and more than offsets the expected reduction of wear rate due to the increase of contact area. This follows since there is a differential in rolling diameters between parts of the contact area of the same wheel.
"Taping line" is a term used in the art to denote the contact point on the wheel-tread when the wheelset is in its central position. In practice on the side of the taping line remote from the wheel flange, often referred to in the art as the "field side", the tread normally, but not essentially, has a straight taper of a suitable conicity which is the same as the inclination of the rail, such as 1:20 or 1:40 as used in practice, while on the other side of the taping line, often referred to as the "gauge side", the tread is concave. On lateral deflection of the wheelset the combined effect of one wheel on the straight tapered portion and the other wheel on the concave portion provides the prevailing conicity for the wheel-tread at the particular lateral deflection and this is referred to as the "effective conicity" at that deflection.
It is therefore an object of this invention to provide a profile for a railway wheel tread which ameliorates the conflict between curving ability and hunting stability mentioned above. In a development, this invention also seeks to provide such a profile that wheel tread wear is minimised.